In cellular radio systems, each cell is a local geographic region containing a base station and a plurality of mobile users. Each mobile user communicates directly with a base station only; there is no direct mobile-to-mobile communication. The base station performs, among other things, a relay function allowing a mobile user to communicate with a user in another location. So, for example, the base station provides coupling of a mobile user's transmission to another mobile user in the same cell, to another base station for coupling to a mobile user in another cell, or to an ordinary public switched telephone network. In this way, a mobile user can send and receive information to and from any other addressable user. It is also to be understood that the term transmitter as used herein can mean either the base station or the user's mobile communication device. Similarly, the term receiver as used herein can mean either the base station or the user's mobile communication device.
Direct Sequence Spread Spectrum (DS-SS) systems, such as Direct Sequence Code Division Multiple Access (DS-CDMA) systems, are attracting widespread attention in the personal communication fields, such as, for example, digital cellular radio. In a DS-CDMA communication system, both the time and frequency domains may be shared by all users simultaneously (this simultaneous sharing of time and frequency domains is to be distinguished from time-division and frequency-division multiple access systems, TDMA and FDMA, where multiple user communication is facilitated with use of unique time slots or frequency bands, respectively for each user). As such, a base station may simultaneously transmit distinct information signals to separate users using a single band of frequencies. Individual information signals simultaneously transmitted may be isolated by each receiving user because of the base station's utilization of unique signature sequences in the transmission of the information signals. Prior to transmission, the base station multiplies each information signal by a signature sequence signal assigned to the user intended to receive the signal. To recover a transmitted signal from among those signals transmitted simultaneously in a frequency band, a receiving mobile user multiplies a received signal (containing all transmitted signals) by its own unique signature sequence signal and integrates the result. By so doing, the user identifies that signal intended for it, as distinct from other signals intended for other users.
In wireless communication systems (such as DS-CDMA systems), an information signal is communicated from a transmitter to a receiver via a channel comprising several independent paths. These paths are referred to as multipaths. Each multipath represents a distinct route an information signal may take in traveling between transmitter and receiver. An information signal communicated via such routes or multipaths, appears at a receiver as a plurality of multipath signals, one signal for each multipath.
The amplitudes and phases of signals received from a transmitter through different multipaths of a communication channel are generally independent of each other. Because of complex addition of multipath signals, the strength of received signals may vary between very small and moderately large values. The phenomenon of received signal strength variation due to complex addition of multipath signals is known as fading. In a fading environment, points of very low signal strength, or deep fades, are separated by approximately one-half wavelength from each other.
Multipaths encountered in wireless communication systems can be described by certain characteristics, such as amplitude attenuation and phase shifting. For example, the multipaths of a DS-CDMA channel may provide different amplitude attenuations and phase shifts to an information signal communicated from a transmitter to a receiver. These different amplitude and phase characteristics may vary due to, e,g., relative movement between transmitter and receiver, or changes in local geography of the transmitter or receiver due to movement. Because of the variation of multipath characteristics, a receiver can experience a signal which fades with time. This fading is a manifestation of the complex addition of multipath signals having time varying amplitudes and phases.
If the characteristics of a DS-CDMA multipath vary slowly, a receiver experiencing a deep fade may observe a weak signal for a long period of time. Long fades are not uncommon in, e.g., indoor radio systems, where relative movement between receivers and transmitters is slow or non-existent (often, one of these two is an immobile base station; the other is a mobile device carried by a person). Since the duration of a deep fade may be large in comparison to the duration of information symbols being communicated, long bursts of symbol errors may occur (due to the weakness of received signal strength for an extended period of time).
To avoid or mitigate the detrimental effects of fading, a technique providing diversity may be employed. Diversity refers generally to the ability of a communication system to receive information via several independently fading channels. As a general matter, diversity techniques enhance a system receiver's ability to combine or select (or both) signals arriving from these independently fading channels, thus enabling (or facilitating) the extraction of communication channels.
U.S. Pat. No. 5,289,499 to Weerackody, the teachings of which are incorporated herein by reference as if fully set forth herein, describes a diversity scheme for a DS-SS system. The system described in this patent utilizes multiple transmit antennas in a digital data communication system to provide diversity by introducing a sequence of distinct weights to segments of a signal to be transmitted. In the illustrative embodiment provided, M copies of the signal are formed where M is the number of antennas used in transmitting the signal. This gives rise to time diversity at the DS-SS receiver. In contrast, the instant invention utilizes multiple antennas placed at the receiver to create time diversity at the receiver.
Also of interest is U.S. Pat. No. 5,305,353 to Weerackody, the teachings of which are incorporated herein by reference, as if fully set forth herein. This patent also teaches the use of multiple antennas at a transmitter, but does not consider the use of multiple antennas at a receiver for use in a DS-SS system.
An article entitled "A Fading Reduction Technique Using Interleave-Aided Open Loop Space Diversity For Digital Maritime-Satellite Communications," by H. Iwai, et al, IEICE Transactions, Vol. E 74, No. 10, October, 1991, pp. 3286-3294, discusses time varying phase weights at a receiver antenna. However, this reference does not discuss a DS-SS system of the type discussed in the instant application.